Aromatic hydroxycarboxylic acids are used as chemical intermediates for the synthesis of drugs and antimicrobials, as well as monomers for preparing polyesters. For instance o-hydroxybenzoic acid (OHBA, salicylic acid) is used as a chemical intermediate, for instance to make aspirin, while p-hydroxybenzoic acid (PHBA) is used to make parabens and is also used as a monomer in making polyesters. They can be prepared using alkali metal aryloxides.
Alkali metal aryloxides are usually prepared by the reaction of an aryl hydroxy compound such as phenol with an alkali metal containing base, such as sodium or potassium hydroxide. This may be done in the presence of water, and in addition water is produced in the reaction.
These aryloxides are useful as chemical intermediates for a variety of chemical processes such as the Kolbe-Schmitt process to make hydroxy-carboxylic acids, see for instance, A. S. Lindsey, et al., Chem. Rev., vol. 57, p. 583-620 (1957). For some of these processes it is desirable to remove essentially all of the water from the aryloxide. This is typically done by heating the solid aryloxide (or initially an aqueous solution of the aryloxide which eventually becomes solid) while applying a vacuum and/or passing an inert dry gas over the aryloxide. However, this is quite time-consuming, and therefore expensive, and thus improved drying methods would be advantageous.
After the alkali metal aryloxide is dried it may be reacted with carbon dioxide to (usually) form a dialkali metal salt of aromatic hydroxycarboxylic acid which may be converted to the free hydroxyacid by reaction with a strong acid, such as sulfuric acid. This is usually done by exposing powdered alkali metal aryloxide to carbon dioxide for long periods of time, often in an apparatus in which the solid (or the resulting paste which is formed during the reaction) may be ground in order to expose new solid surface for reaction with the CO.sub.2. Thus, improved carboxylation procedures for the Kolbe-Schmitt reaction are of commercial interest.
Canadian Patent 881,906 and U.S. Pat. No. 3,554,265 describe a process for drying alkali metal phenates in a spray drying column in which the drying gas enters the column at 250 to 500.degree. C. No mention is made that the temperature of the alkali metal phenoxide reaches above the melting point of the phenoxide, nor that the dry phenoxide is ever a liquid.